As well known to persons skilled in the art, when a semiconductor is ground or cut, a plurality of semiconductor wafers are stored in a cassette, and the cassette is carried to a predetermined site. The cassette has a pair of side walls extending vertically at an interval therebetween in the crosswise direction, and in the inner sides of the side walls, support ribs or support grooves are formed with a space therebetween in the vertical direction. The semiconductor wafers are each stored in the cassette by positioning their both edge portions on the respective support ribs or in the respective support grooves of both the side walls. Therefore, a plurality of semiconductor wafers are stored with a space therebetween in the vertical direction (the space corresponds to an interval between adjacent support ribs or support grooves) in the cassette. An adsorber having suction holes in its top surface is used to carry semiconductor wafers into the cassette or to carry them out from it. To carry a semiconductor wafer into the cassette, the semiconductor wafer is vacuum adsorbed to the adsorber and carried to a predetermined position in the cassette, the vacuum adsorption of the semiconductor wafer is then released to leave the semiconductor wafer in the cassette, and the adsorber is pulled out from the cassette. To carry a semiconductor wafer out from the cassette, the adsorber is put into the cassette, positioned under the semiconductor wafer to vacuum adsorb the semiconductor wafer, and then, the adsorber suction-holding the semiconductor wafer is pulled out from the cassette.
The above conventional cassette, however, involves the following problems. That is, the thickness of each semiconductor wafer is often made extremely small, for example, to 100 μm or less, particularly 50 μm or less, in recent years. When a semiconductor wafer having an extremely small thickness is stored in the cassette, it may be bent in a concave form so that its center portion is liable to be displaced in a downward direction. Since the curvature of each semiconductor wafer is not always the same, when the semiconductor wafer bends, the space in the vertical direction between adjacent semiconductor wafers stored in the cassette with a space therebetween in the vertical direction becomes extremely small locally. Therefore, there is a possibility that when the adsorber is inserted into the cassette and positioned under the semiconductor wafer, it may bump into the semiconductor wafer and damage the semiconductor wafer. Further, there is a possibility that when the semiconductor wafer greatly bends, it may come into contact with another semiconductor wafer arranged adjacent thereto in the vertical direction, whereby the semiconductor wafer may be damaged.
JP-A 2000-91400 discloses a cassette provided with a plurality of support plates arranged with a space therebetween in the vertical direction to define a plurality of semiconductor wafer storage spaces in the cassette. Each of the support plates has a cut-out correspondent to the shape of an adsorber for suction-holding a semiconductor wafer. In the cassette, each semiconductor wafer is supported not only at its both edge portions but also at its center portion, thereby suppressing its curvature caused by its own weight. As semiconductor wafers adjacent to each other in the vertical direction are separated from each other by the support plate, they are not contacted to each other.
However, the cassette disclosed by the above JP-A 2000-91400 remains yet not satisfactory. According to the experience of the inventor of the present invention, a semiconductor wafer which has been made very thin by grinding its back surface is bent in a concave or convex form due to grinding distortion or crystal orientation. Therefore, even when the semiconductor wafer is supported at its both edge portions and its center portion, the semiconductor wafer is curved. Accordingly, even when the cassette disclosed by JP-A 2000-91400 is used, there still exists a possibility that the adsorber may bump into the semiconductor wafer and damage it, at the time when the adsorber is inserted into the cassette.